To see fundraising and spending activity in current primaries, visit CFI's website.

In the email interview Boatright noted that the Virginia race was interesting in that "there was so little activity by outside groups. Most of the strongest ideological challenges over the past decade involved organizations like the Club for Growth, Citizens United and the Tea Party Express (on the right) and MoveOn.org and labor unions (on the left)."

He also noted that incumbents defeated in primaries is not a growing trend. "Over the past three election cycles more representatives have faced primary opponents, but the success rate of challengers has been quite consistent. The last two election cycles have seen a resurgence of primary competition. There has, however, been no change in the number of defeated incumbents. Every year (redistricting years excepted) three or four incumbents are defeated in their primaries."

Clark University political scientist Robert Boatright just published “Getting Primaried,” which is now the definitive work on congressional primary elections. He answered several questions about Eric Cantor’s defeat via e-mail.

Is Eric Cantor’s loss part of a trend? Are more representatives losing primary election battles?

His loss is not part of a trend. Over the past three election cycles more representatives have faced primary opponents, but the success rate of challengers has been quite consistent. The last two election cycles have seen a resurgence of primary competition. There has, however, been no change in the number of defeated incumbents. Every year (redistricting years excepted) three or four incumbents are defeated in their primaries.

Many people are interpreting Cantor’s loss as an ideological statement by conservatives. Are most primary challenges or victories about ideology?

It is rare to see a victorious primary challenger who has run explicitly on ideology. In my book I categorized House and Senate primary challenges since 1970. Only 19 percent of cases involved a conservative challenge to a Republican or a liberal challenge to a Democrat. Nearly 40 percent of primary challenges have to do with the incumbent’s age, incompetence, or ethical troubles. In fact, people who challenge incumbents for reasons other than ideology fare better than those who explicitly run on ideology.

There has been an increase in ideological challenges since 2010, which likely has much to do with the tea party but also with an increase in challenges to Democrats from the left. I have speculated that this trend is due to turnover in Congress. In wave elections like 2010, there is increased primary competition within the party that benefits from the wave.

But whether challengers were really motivated by ideology is probably less important than the fact that the media takes their victories to be about ideology. That is, if enough people see the race as being a referendum on ideological moderation, the public (and more importantly, other members of Congress) will take the race as being about ideology, and will behave accordingly.

What stands out to you about Cantor’s loss, in light of your book’s findings?

Cantor’s defeat is interesting because there was so little activity by outside groups. Most of the strongest ideological challenges over the past decade involved organizations like the Club for Growth, Citizens United and the Tea Party Express (on the right) and MoveOn.org and labor unions (on the left). These groups typically concentrate their efforts on one or two primaries, often in small states or states with unusual primary laws, like Utah. In other words, groups have used a small and select number of primaries to show that they can take down incumbents.

Cantor’s defeat does not fit this pattern. During the 2014 primaries, I have been working with the Campaign Finance Institute to track independent expenditures. Although outside groups have spent substantial money in this year’s primaries, Cantor’s opponent, David Brat, had received almost nothing. Less than $5,000 was spent by outside groups on his behalf. There are a few instances over the past decade where incumbents were defeated by opponents who spent very little, but there are none where an incumbent was defeated by an ideological opponent who spent so little and had so little outside support.

Is there any reason to think that his loss will engender more primary challenges in future elections?

This race may well inspire the remaining primary challengers out there in 2014. There is a lot of interest group money available to conservative challengers, and one or two of these candidates may seek to position himself as the next David Brat.

But beyond 2014? Here, you need to make a distinction between competitive and uncompetitive primaries. Most primary challengers receive few votes and in many states whether any primary challenger emerges has more to do with ballot access rules. For instance, Maryland and Indiana require few signatures to get on the ballot so a lot of people run, but these aren’t really credible candidates.

So, after Cantor’s loss, more people may run, but it is hard to imagine more competitive candidates running. I think the Republican Party in particular is already at a saturation point. If Eric Cantor is sufficiently moderate to merit a challenge, it’s hard to imagine that there are very many Republicans who merit a challenge from the right but haven’t already gotten one.

What about primary challenges among Democrats?

Although some have claimed that the Democrats have more primary challenges than Republicans, Democratic primaries in recent decades have mostly been the remnants of one party competition in the South or the consequences of creating majority-minority districts. If one looks solely at elections since 1992, Democratic primaries are less competitive than Republican ones.

What do you make of the fact that Cantor lost even though he was the majority leader?

Incumbents in leadership positions or who run for higher office have historically been at risk of primary challenges. Presidential contenders (John Kerry, John McCain, Joe Lieberman, Ron Paul, even Dennis Kucinich) often are challenged on the basis that they have lost touch with their states or districts. Party leaders like Mitch McConnell, John Boehner and Cantor have also faced challenges. If so, this suggests that being in a leadership position may make one more vulnerable than in the past.

In sum, what lessons, if any, should political observers learn from Cantor’s loss?

As I noted above, it is easy to allow perception to become reality. Primaries provide an irresistible story line to hang arguments about ideological extremism or about the popularity of issues like immigration or guns.

There are, however, a lot of idiosyncrasies here. Virginia does not have a history of competitive primaries. In fact, only three times since 1970 has a Virginia incumbent been held to less than 75 percent of the primary vote. There were no statewide primary races on the Republican ballot, so there was little reason to turn out to vote other than to send a message to Eric Cantor. Cantor’s district had been redrawn in 2012, and there’s been some speculation that his duties in Congress kept him from getting to know his new constituents very well. It is hard to contend that an unrepresentative sample of 60,000 Virginians says very much about national politics, any more than does the fact that similarly small numbers of people reelected allegedly endangered Republicans in states such as South Carolina.

Every year there are several hundred House primaries that each play out before small electorates. It’s inevitable that a few of them will provide a compelling story line — an incumbent caught napping, or some other “message.” If we see plenty of articles that allege that the Cantor defeat means that immigration reform is dead or some other such grand policy implication, then the story may become reality. But that reality doesn’t follow from the election itself.